1. Field of the Invention
This invention relates to transceiver circuits for computer networks. In particular, it relates to the type of transceiver that is used to interface a communications node of a computer network to the network for receiving and transmitting digital signals on the network. More specifically, the invention relates to the way such transceivers are protected from over-voltage and over-current conditions when the communications node is powered-down.
2. Prior Art
Computer network communications node transceivers are well known in the art. Typically, these transceivers are built from circuits that are integrated onto a semiconductor chip. A block diagram of one such prior art transceiver chip is shown as part of FIG. 1. A transceiver 100 comprises a receiver RX, a transmitter TX, and logic circuitry 101 to control the operation of the transmitter and receiver and communicate with the processor in the node in which the transceiver is installed. The transmitter has a pair of output terminals and the receiver has a pair of input terminals. The transmitter receives from the logic circuitry a control signal A and its compliment AN. The transceivers of FIG. 1 are shown coupled to a point-to-point communications network 102 so they can communicate with each other. Typically the transceivers are coupled through a transformer and a terminating resistor; these elements are omitted for simplicity.
An example of one type of network in which communications node transceiver chips are employed is the Integrated Services Digital Network or ISDN. The internationally recognized specification for the standard 4-wire ISDN interface is CCITT Recommendation I.430. According to this specification, a multi-drop ISDN network can be configured with a number of full-function ISDN stations or nodes called "NT's" (network termination) and a large number of lesser function stations or nodes called "TE's" (terminal equipment). The NT's can transmit to many other TE's and receive from many other TE's, whereas the TE's can only transmit to an NT and receive from an NT. The transceivers of such a multi-drop ISDN and their interconnection are shown in FIG. 2. The NT transceiver 105 transmits to the TE transceivers 106 on the wire pair 103 and the NT transceiver 105 receives from the TE transceivers on wire pair 104. Once again, coupling transformers and terminating resistors which would normally be external to the transceiver chips but within the communications nodes are not shown for simplicity. The control logic within the transceiver chips is also not shown for simplicity.
An important part of the ISDN specification is related to powered-down loading. To prevent a TE which is powered down from loading down the NT transmit lines, a maximum allowable current that can be drawn from the network by a powered-down TE is specified. For a 1.2 Volt peak value 96 kHz signal the current must be no greater than 0.5 mA peak. Since prior art ISDN transceivers had separate terminal pairs for transmit and receive, a powered-off protection circuit to enable an ISDN transceiver to meet this specification had to take into account primarily only the receiver circuit characteristics.
FIG. 5 shows the detail of a typical prior art powered-down protection scheme in a transceiver 100 like that shown in FIGS. 1 and 2. Typically, the receiver is interfaced to the network through a 2:1 turns ratio coupling transformer 15. This means that 2.4 volts is applied to the receiver (1.2 volts times 2) during powered-down testing. For most ISDN transceivers, overload protection against electrostatic discharge (ESD) is desired. This overload protection is usually provided by ESD protection diodes PD integrated into the chip and connected between the receiver inputs, the power supply rail V.sub.DD, and ground. If the receiver input terminals were to be connected directly to the coupling transformer which is in turn connected to the network, the ESD protection diodes would be forward biased when the power to the transceiver is off (i.e. V.sub.DD =0) and a voltage with magnitude greater than a diode drop is present across the input terminals; thereby drawing too much current for the receiver to meet the ISDN powered-down specification. To provide powered-down protection for the receiver, current limiting resistors R0 and R1 are placed in the line between the receiver input terminals and the transformer. Since the diodes connected to the power supply do not conduct until the voltage across them is at least 0.7 V, depending on the technology used, the resistors limit the current enough to meet the specification when the power supply voltage V.sub.DD goes to 0 and a peak voltage of 2.4 V is present on the receiver input. The transmitter output terminals are connected to a separate 2:1 transformer 13 and may also be connected to ESD protection diodes. No current limiting is needed for the transmitter because it is separate from the receiver.
While the above scheme works well and provides ISDN communications to many users satisfactorily, it has a major disadvantage when it comes to configuring a network. Since each transceiver has separate ports for receive and transmit, a user must know which set of wires is which and keep them straight over the entire network. Also, if a user wishes to change a TE type node to an NT type node or vice versa, wiring changes must be made. It would be much more desirable to use so-called "single-port" transceivers for an ISDN network; that is transceivers with a single set of input/output terminals that both transmit and receive signals.
Single-port transceivers are already known in the art. FIG. 7 shows a simplified block diagram of a typical single-port network communications transceiver 300. The transmitter TX and receiver RX are connected to the input/output terminals 305 of the transceiver 300 in parallel. A multiplexer 302 is controlled and fed signals by logic circuits 303. The multiplexer alternatively activates either the receiver or the transmitter to put the transceiver in either receive mode or transmit mode under software or microcode control. The transceiver is coupled to a single set of network wires through the transformer 310.
With a single-port transceiver, an ISDN network could be configured as in FIG. 8 with each transceiver 300 connected to the network 500 in an identical fashion. The selection of which nodes are to act as NT's and which as TE's could be changed quickly, without re-wiring. Such a scheme would greatly increase the flexibility of network connections and layout.
Unfortunately, single-port transceivers have seldom been employed with ISDN because of the powered-down protection requirements of the ISDN specification. With the transmitter and receiver in parallel, the resistors previously discussed will not work because the resistors would be in the output lines while the transceiver is transmitting and would attenuate the signal too much to drive the network adequately. With the resistors removed, not only do ESD protection diodes draw too much current in the powered-down state to meet the ISDN specification, but the transmitter would also draw current because of the inherent design of the typical transmitter output circuit, discussed below.
The typical transmitter output circuit as shown in FIG. 3 has an input for a reference current, I.sub.REF at 11. The reference current can be supplied in any number of well-known ways. In this example the current source is connected in series with a voltage clamp consisting of the operational amplifier 18 and the p-channel transistor Q5. The circuit itself consists of two transistor pairs, the first consisting of Q1 and Q3 in a pull-up, pull-down arrangement that is well known, and the second consisting of Q2 and Q4 in the same arrangement. The circuit is driven by the control logic 101 of FIG. 1 with a control signal A and its compliment AN. The output terminals 14 drive the transformer 13 of FIG. 3. In the usual arrangement, the pull-up and pull-down transistors are the normally-off, n-channel p-well type fabricated on an n type substrate. As is well known in the art, the well terminal of each transistor must be tied to a low potential in order to ensure that the well remains at a lower voltage than the drain so that current can flow when the transistor turns on. In practice, the well terminal of the transistor is usually tied to the source as with the connections 12 of FIG. 3. To ensure the well-to-substrate junction does not become forward biased, the substrate is normally tied to V.sub.DD. A cross section of a transistor 20 showing fabrication detail and these connections is shown in FIG. 4. Because of this arrangement, when power is cut off from the chip and V.sub.DD goes to 0 volts, voltage on the input/output terminals causes the well-to-substrate junction of the pull-up transistors to be forward biased, and so the transmitter draws current from the network in excess of the specification.
In order to solve similar problems, over-voltage and over-current protection schemes used in other types of networks where single-port transceivers are employed have often involved mechanical relays, as is the case for the circuit disclosed in U.S. Pat. No. 4,709,296 to Hung et al. Such relays take up space outside the transceiver chip and have low reliability. Solid state protection circuits have been developed to replace the relay, but until now such circuits have been fairly complex and have had too many components to allow easy integration onto the transceiver chip. Such a solid state protection circuit is disclosed in U.S. Pat. No. 5,142,429 to Jaki.
What is needed is a powered-down protection scheme that would require only a very simple circuit with few components in addition to those already present in a typical transceiver design. Such a scheme would allow the protection circuitry to be easily integrated onto the transceiver chip so that single-port transceivers can be used with ISDN networks.